The United States government has rights in this invention by virtue of NIH grants DK-19813 and HD 34149 and Yale Core Center for Musculoskeletal Disorders P30 AR46032.
Glucose-dependent Insulinotropic Peptide (GIP) is a 42 amino acid peptide synthesized and secreted from endocrine cells in the small intestine. GIP""s role in coupling nutrient intake and insulin secretion, the xe2x80x9cincretinxe2x80x9d effect, is well known. Parathyroid hormone and vitamin D are known to couple calcium intake to bone formation but no coupling hormone has been identified for nutrient intake and bone formation.
GIP is secreted from enteric endocrine cells in the proximal small intestine, whereas glucagon-like peptide 1 (GLP-1, another major xe2x80x9cincretinxe2x80x9d hormone) is secreted from endocrine cells in the terminal small bowel. Until recently, the glucose-dependent insulinotropic peptide (GIP) had been considered a parochial hormone of the enteric endocrine system with its major site of action being the xcex2-cells of the endocrine pancreas. R. A. Pederson, et al., Endocrinology 99, 780-785 (1976). However, the cloning of the GIP receptor led to the discovery that the receptor is expressed in a wide range of tissues and organs, including the exocrine pancreas, and distal small cells in several vascular beds. T. B. Usdin, et al., Endocrinology 133,2861-2870 (1993).
This widespread receptor distribution suggests as yet undefined physiological actions of GIP. Most studies seeking to define the actions of GIP have focused on synergism between GIP and glucose in stimulating insulin secretion. GIP infusions have also been shown to inhibit effects of glucagon on the liver while enhancing those of insulin, and to have dual effects on hepatic blood flow, increasing flow through the portal vein and inhibiting flow through the hepatic artery. Ironically, the effect for which GIP was discovered, inhibition of gastric acid secretion, seems to be a minor pharmacological effect of little physiological significance.
At present the main modulator of bone metabolism is thought to be the PTH-Vitamin D axis. Parathyroid hormone is known to be negatively regulated by nutrient absorption, with PTH secretion decreasing after a calcium-rich meal and rising during fasting W. Jubiz, et al., J Clin Invest 51, 2040-2046 (1972). Receptors for PTH are found on osteoblasts and PTH-induces cytokine expression which in tun modulates osteoclastic activity, J. E. Onyia, et al., J Cell Biochem 67, 265-74 (1997), J. H. Pollock, et al., J Bone Miner Res 11, 754-9 (1996). Thus, PTH-induced bone turnover is generally a coupled process and, in conjunction with Vitamin D, PTH plays a major role in bone mineralization.
In addition to calcium intake, however, bone depends for its growth and remodeling on nutrient intake. In fact, even in a state of high bone turnover, such as in an ovariectomized rat, it is possible to prevent bone loss by altering the rat""s diet and placing her on a specified protein diet, B. H. Arjmandi, et al., J Nutr 126, 161-167 (1996), suggesting that a gut-induced signal may modulate bone turnover. To date, the hormones of the enteric endocrine system have not been considered to play a major role in coordinating nutrient intake with skeletal growth and remodeling.
It is therefore an object of the present invention to provide a means for regulating skeletal growth and remodeling.
It is a further object of the present invention to provide therapeutic formulations for treatment of disorders such as osteoporosis.
The examples demonstrate that GIP receptor mRNA and protein are present in normal bone and osteoblastic-like cell lines, and that high-affinity receptors for GIP can be demonstrated by 125I GIP binding studies. When applied to osteoblast-like cells (SaOS2), GIP stimulated an increase in cellular cAMP content and in intracellular calcium, with both responses being dose dependent. Moreover, administration of GIP results in elevated expression of collagen type I mRNA as well as an increase in alkaline phosphatase activity. Both of these effects reflect anabolic actions of presumptive osteoblasts. These results provide the first evidence that GIP receptors are present in bone and osteoblastic like cells and that GIP modulates the function of these cells.
GIP has anabolic actions on remodeling bone, increasing vertebral bone density in a rat model of osteoporosis. GIP at 10 nM inhibits PTH-induced bone resorption in a fetal long bone assay and stimulates the synthesis of type 1 collagen mRNA. Transgenic mice overexpressing GIP have increased bone density compared to same age controls.
GIP or analoges thereof can therefore be used as a therapeutic to inhibit bone resorption and to maintain or increase bone density. GIP antagonists, compounds which block binding to the GIP receptor, can be used to decrease bone density.